In some environments where a large and heavy object is moved and there is a desired position at which the object is to be stopped, or a border which must not be passed, distance measurements by means of radar technology has proved to be useful. Typically, a transceiver device (transmitter and receiver) is arranged on the moving object, and a reflector device is arranged at a fixed reference position. By transmitting a radar signal at radio frequency, i.e. an RF signal, and detecting the reflection thereof it is possible to determine the distance between the transceiver and the reflector.
In many applications there are relatively high demands on the accuracy of the distance determination. In one accurate method that has been developed the frequency of the RF signal is swept over a sweep range, i.e. a frequency range, by discrete frequency steps, while being continuously transmitted. At each frequency the phase difference between the transmitted signal and the received reflected signal is determined. The frequency at each step is maintained long enough to allow the transmitted signal to return after reflection. This method is called stepped frequency continuous wave (SFCW) radar. By sweeping the sweep range stepwise, and detecting, for each frequency, the phase difference between the reflected signal and the transmitted signal, it is possible to determine the distance. Depending on the frequency of the RF signal the distance between the transceiver and the reflector constitutes a number of wavelengths, which also corresponds to a number of periods of the RF signal. With increasing frequency the number of periods will increase for the same distance, and the phase difference will change. Since the distance typically corresponds to several full periods plus a portion of a period, and the phase difference only gives information about that portion of a period, a single frequency measurement is not enough to determine the distance. By means of several phase difference measurements at different frequencies it is however possible to determine also the correct number of full periods, and thus the distance.
However, the transmitted radar signal has a certain physical width, meaning that usually many reflections are received from objects around the reflector. There is a problem in determining which one of the reflections originates from the reflector. Many efforts have been made during the years to provide an efficient solution. One example thereof is disclosed in WO 01/23906, which deals with CW radar signals in general. The transmitted RF signal is modulated at the reflector. By means of the modulation it is possible to suppress the unwanted reflections and easier filter out the reflection of interest. In WO 01/23906 it is suggested that the modulation can be an amplification, i.e. at the reflector the transmitted signal is amplified and retransmitted, rendering the amplitude of the received retransmitted signal significantly higher than the amplitude of the other reflections. This is however still not very reliable. Other proposed modulations, which enhances the possibility of identifying the reflection of interest, are frequency modulation and amplitude modulation.
For stepped frequency continuous wave distance measurements, as explained above, the phase difference between the transmitted signal and the received reflected signal is determined. The phase detector outputs a value that is related to cosines or sinus of the phase difference, which means that it outputs the same value for two different phase differences, one between 0 and 180 degrees and one between 180 and 360 degrees. The modulation of the signal at the reflector affects the phase of the signal. At the receiver there is no synchronization with the modulator at the reflector, which means that there is no phase information about the modulation as such. This causes an ambiguity as to which contribution the modulator brings at each point in time. Consequently, in order to make an accurate determination of the distance, it must be known at each point in time what is the phase contribution from the modulator.